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Rent growth for U.S. multifamily fell to 2% year-over-year in May from 2.5% in April, according to Yardi Matrix’s Matrix Monthly report, marking the steepest YOY rent-growth decline in more than six years.

Oversupply is at the root of May’s numbers, which reflect a two-year pattern of rent deceleration, according to Yardi. More than 600,000 new units are under construction and expected to be delivered over the next two years, putting pressure on rent and occupancy, even in high demand markets.

Only two major metros posted rent growth above 4% YOY for May: Orlando (5.3% YOY) and Las Vegas (4.9% YOY), followed by Sacramento and Tampa at 4%.

Places such as Austin, Portland, OR, and Seattle—once considered high-growth markets—have seen rents decelerate and even turn negative because of a glut of new supply. Portland and Seattle recorded rent growth of 0.4% and 0.5% respectively, while Austin reported negative rent growth of -0.5%.

The high number of deliveries will likely keep rent growth muted, according to Yardi. The impact is expected to be particularly strong in markets with an oversupply of new luxury apartments, which account for 90% of new construction. Lifestyle (or luxury) rents are negative on a YOY basis in Seattle, Portland, Nashville, Austin, and Washington, D.C.—all high-growth, high-delivery metros.

More than 300,000 new units are expected to be delivered in 2018, most of them luxury units. The oversupply of luxury units created a wide spread between Lifestyle rent growth rates (1.2% YOY) and Renter-by-Necessity rent growth rates (2.6% YOY), according to Yardi data.